Hoo boy! That's a BIG ask!
Does he want to go right back to the dawn of time, or just the Middle Ages? Are we including the Ancient Greeks and Romans in this? How much detail does he want? Is he interested in the structure and workings of the armour, how it was tied on, good and bad points etc etc? What about Oriental and Middle Eastern armour?
There are various good books and websites around, but it depends on what exactly you want to cover.
I'll give you a start, by pointing you at the following websites:
http://www.gothiceye.com/pictures.asp?c ... &offset=72 - a good collection of effigies and rubbings from the brass plaques placed on the graves of knights from about 1200 to the late 15th century
http://www.renfroana.150m.com/liberadhonorem.htm - Italian, under the German Emperors in about 1195 (but pretty typical of most of Western Europe).
http://www.callisto.si.usherb.ca/~crois ... images.htm - an interesting mix, from a variety of periods. Don't take any notice of the dates in bold type - they just tell when the incident depicted
happened, not when the picture was painted. To get the actual time period of the picture, look at the stuff at the end of the entry, where it says XIIIe siecle or whatever - that means 13th century (ie from 1201 to 1300).
http://www.keesn.nl/mac/mac_en.htm contains pics from about 1250 from two sources - the (French) Maciejowski Bible and the (German) Codex Manasse.
For Byzantine stuff, look at
http://www.levantia.com.au/ - this relates specifically to the 11th and 12th centuries, but is fairly typical.
For the Vikings up till the 11th century, try Regia Anglorum
http://www.regia.org/Mail.htm or The Vikings websites.
For 7th century north-west Europe, see
http://www.havenonline.com/moas/northst ... /armor.htm
For Ancient Greece, see
http://www.larp.com/hoplite/index.html - or if you want to go back to even before that (say about the time of the siege of Troy), see the dendra panoply at
http://home.att.net/~a.a.major/mycarmor2.jpg - bronze armour, helmet made of boar tusks and all. This is an isolated piece, though - there's no way of knowing how common it was. Maybe there was only ever one example.
For Romans, you could look at
http://www.esg.ndirect.co.uk/ or
http://rubens.anu.edu.au/student.projec ... index.html or
http://www.larp.com/legioxx/squamata.html
For mediaeval Russian armor, look at
http://www.strangelove.net/~kieser/Russia/armor.html
For the armour of the Ottoman Turks of the 15th and 16th centuries, see
http://www.turkishculture.org/weapons/armor.html
Western Europe 15th century? See
http://www.theredcompany-1471.org/ or
http://www.theredcompany-1471.org/
See what I mean? There's
plenty of information out there - depends what you want, where and when.
